1. Field of the Invention
The present invention relates to a reversible thermosensitive coloring composition comprising an electron-donating coloring compound and an electron-accepting compound, capable of inducing color formation by utilizing the coloring reaction between the electron-donating compound and the electron-accepting compound. The present invention also relates to a reversible thermosensitive recording medium using the above-mentioned reversible thermosensitive coloring composition, which recording medium is capable of repeatedly forming a colored image therein and erasing the same therefrom by controlling the thermal energy applied thereto.
2. Discussion of Background
There is conventionally known a thermosensitive recording medium utilizing the coloring reaction between an electron-donating compound (hereinafter referred to as a coloring agent) and an electron-accepting compound (hereinafter referred to as a color developer). This kind of thermosensitive recording medium is widely used, for example, for a facsimile apparatus, a word processor, and a printer of a scientific measuring instrument. However, the coloring reaction of the conventional thermosensitive recording medium of this type has no reversibility, so that color development and decolorization cannot be alternately repeated.
Among published patents, there are several proposals for a thermosensitive recording medium which can reversibly carry out the color development and decolorization utilizing a coloring reaction between a coloring agent and a color developer. For example, a thermosensitive recording medium using phloroglucinol and gallic acid as color developers in combination is disclosed in Japanese Laid-Open Patent Application 60-193691. There is disclosed in Japanese Laid-Open Patent Application 61-237684 a reversible thermosensitive recording medium which employs compounds such as phenolphthalein and thymolphthalein as color developers. In Japanese Laid-Open Patent Applications 62-138556, 62-138568 and 62-140881, there are disclosed reversible thermosensitive recording media, each comprising a recording layer which contains a homogeneously dissolved composition of a coloring agent, a color developer and a carboxylic acid ester. Further, in Japanese Laid-Open Patent Application 63-173684, a reversible thermo-sensitive recording medium comprising as a color developer an ascorbic acid derivative is disclosed. In addition, a reversible thermosensitive recording medium comprising as a color developer a salt of bis(hydroxy-phenyl) acetic acid or gallic acid, and a higher aliphatic amine is disclosed in Japanese Laid-Open Patent Applications 2-188293 and 2-188294.
In the above-mentioned conventional reversible thermosensitive recording media, however, both of the stability of a colored image and the facility for decolorization are not always satisfied at the same time, and there is the problem with respect to the density of a colored image, and the stability of the repeated coloring and decolorization operations. Therefore, the conventional reversible recording media are not satisfactory for practical use.
The inventors of the present invention have previously proposed a reversible thermosensitive coloring composition which comprises an organic phosphoric acid compound, an aliphatic carboxylic acid compound or a phenol compound, each having a long-chain aliphatic hydrocarbon group therein, as a color developer, and a leuco dye as a coloring agent, as disclosed in Japanese Laid-Open Patent Application 5-124360. By use of such a reversible thermosensitive coloring composition, the color development and the decolorization can be easily carried out by controlling the heating and cooling conditions for the coloring composition, and further, the color-developed state and the decolorized state can be stably maintained at room temperature, and the color development and the decolorization can be alternately repeated in a stable condition. In this application, there is also proposed a reversible thermosensitive recording medium which comprises a recording layer containing the above-mentioned reversible thermosensitive coloring composition. Although this kind of reversible thermosensitive recording medium attains a satisfactory level for practical use with respect to the compatibility of the stability of a colored image with the facility for decolorization, and the density of a colored image, there is yet room for improvement in the problem of preservation stability.
The use of a phenol compound with a specific structure having a long-chain aliphatic hydrocarbon group as the color developer is proposed in Japanese Laid-Open Patent Application 6-210954. However, a reversible thermosensitive recording medium comprising the above-mentioned phenol compound as the color developer has the same drawbacks as mentioned above.